Absolute palaeointensity of Oligocene (28-30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean).

We report palaeointensity estimates obtained from three Oligocene volcanic sections from the Kerguelen Archipelago (Mont des Ruches, Mont des Tempêtes, and Mont Rabouillère). Of 402 available samples, 102 were suitable for a palaeofield strength determination after a preliminary selection, among which 49 provide a reliable estimate. Application of strict a posteriori criteria make us confident about the quality of the 12 new mean-flow determinations, which are the first reliable data available for the Kerguelen Archipelago. The Virtual Dipole Moments (VDM) calculated for these flows vary from 2.78 to 9.47 10e22 Am2 with an arithmetic mean value of 6.15+-2.1 10e22 Am2. Compilation of these results with a selection of the 2002 updated IAGA palaeointensity database lead to a higher (5.4+-2.3 10e22 Am2) Oligocene mean VDM than previously reported, identical to the 5.5+-2.4 10e22 Am2 mean VDM obtained for the 0.3-5 Ma time window. However, these Kerguelen palaeointensity estimates represent half of the reliable Oligocene determinations and thus a bias toward higher values. Nonetheless, the new estimates reported here strengthen the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0.3 Ma

Determination of flow directions combining AMS and thin section analyses : implications for the Oligocene volcanism in the Kerguelen Archipelago (southern Indian Ocean)

We report an Anisotropy of Magnetic Susceptibility (AMS) study carried out on seven Oligocene volcanic sections widely distributed on the surface of the Kerguelen Archipelago. The statistical results at flow scale and at section scale were checked with AMS density diagrams and thin section analysis. Because of axes permutations frequently observed for lava flows, two possible flow directions were nicely defined for the majority of the sections. We then used two alternative approaches (imbrication of the minimum axis and symmetry of the AMS distribution) to infer a flow direction for each section. These two methods and thin section analyses not being decisive for choosing between the two solutions, geological and geomorphological observations were finally used for confident interpretations. The results of this study helped to more precisely locate the supposed eruptive centers of the Kerguelen archipelago

Design Optimisation and Mass Saving of the Structure of the Orion-MPCV European Service Module

This paper presents an overview of the design optimisation measures that have been proposed and analysed in order to reduce the mass of the structure, including the MMOD (Micro-Meteoroid and Orbital Debris) protection system, of the ESM (European Service Module) for the Orion MPCV (Multi-Purpose Crew Vehicle). Under an agreement between NASA and ESA, the NASA Orion MPCV for human space exploration missions will be powered by a European Service Module, based on the design and experience of the ATV (Automated Transfer Vehicle). The development and qualification of the European Service Module is managed and implemented by ESA. The ESM prime contractor and system design responsible is Airbus Defence and Space. Thales Alenia Space Italia is responsible for the design and integration of the ESM Structure and MMOD protection system in addition to the Thermal Control System and the Consumable Storage System. The Orion Multi-Purpose Crew Vehicle is a pressurized, crewed spacecraft that transports up to four crew members from the Earths surface to a nearby destination or staging point. Orion then brings the crew members safely back to the Earths surface at the end of the mission. Orion provides all services necessary to support the crew members while on-board for short duration missions (up to 21 days) or until they are transferred to another orbiting habitat. The ESM supports the crew module from launch through separation prior to re-entry by providing: in-space propulsion capability for orbital transfer, attitude control, and high altitude ascent aborts; water and oxygen/nitrogen needed for a habitable environment; and electrical power generation. In addition, it maintains the temperature of the vehicle's systems and components and offers space for unpressurized cargo and scientific payloads. The ESM has been designed for the first 2 Lunar orbit missions, EM-1 (Exploration mission 1) is an un-crewed flight planned around mid-2020, and EM-2, the first crewed flight, is planned in 2022. At the time where the first ESM is about to be weighted, the predicted mass lies slightly above the initial requirement. For future builds, mass reduction of the Service Module has been considered necessary. This is being investigated, together with other design improvements, in order to consolidate the ESM design and increase possible future missions beyond the first two Orion MPCV missions. The mass saving study has introduced new optimised structural concepts, optimisation of the MMOD protection shields, and optimised redesign of parts for manufacturing through AM (Additive Manufacturing)

Fluctuations du champ paléomagnétisme terrestre enregistrée par les laves de l'Archipel des Kerguelen (partie australe de l'Océan Indien) et détermination de leur direction de mise en place.

Marc DAIGNIERES, Professeur, Université Montpellier II, PrésidentAnnick CHAUVIN, Professeur, Université Rennes I , RapporteurPierre ROCHETTE, Professeur, Université Aix Marseille III , RapporteurPierre CAMPS, Chargé de Recherche, Université Montpellier II ExaminateurBernard HENRY, Chargé de Recherche, IPG Paris, ExaminateurMireille PERRIN, Directeur de Recherche, Université Montpellier II, Directeur de thèseNumerous observations of the magnetic field variations, uniformly distributed on the Earth'ssurface for different time windows are needed to improve our knowledge on the inner-core dy-namic. The first part of this work reports a paleomagnetic study, direction and intensity, carriedout on oligocene lava flows from the Kerguelen archipelago (49° 20'S, 70° 20'E) in order to im-plement international databases. A between-flow angular standard deviation of 21.2 to 21.8°from the geographic pole has been computed. This estimation of the oligocene paleosecularvariation agrees with McFadden et al. (1991) Model G for paleosecular variation between 22.5-40 Ma. The 12 new paleointensity determinations reported are the first estimations obtainedfor this archipelago with good technical quality. The mean VDM of 6.15+-2.1 10e22 increasesthe mean oligocene estimate but is still lower than recent value ( 0.3 Ma), which seems abnormaly high.Moreover, this work brings additional constraints to the geological researches carried out onKerguelen. We proposed a better location of the eruptive center for the different magnetic pro-vinces and a detailed chronology of emplacement for a southeastern site by means of magneticfabric studies. Correlation of the polarity successions for fiv e sections with a global geomagne-tic polarity scale allowed refinement of the emplacement time of these volcanic sequences andconsequently reevaluation of their extrusion rates.La compréhension de la dynamique interne du noyau terrestre nécessite l'observation desfluctuations du champ magnétique en des sites répartis uniformément à la surface du globepour différentes périodes de temps. La première partie de cette thèse présente les résultatsd'une étude paléomagnétique, en direction et en intensité, réalisée sur des laves oligocènesde l'archipel des Kerguelen (49° 20'S, 70° 20'E) en vue de compléter les bases de donnéesinternationales. Celle-ci a permis d'évaluer la dispersion angulaire inter-coulées entre 21.2 et21.8° par rapport au pôle géographique. Cette estimation de la variation paléoséculaire oli-gocène est en accord avec le Modèle G de McFadden et al. (1991) pour la période 22.5-45Ma. Les 12 déterminations de paléointensité reportées sont les premières d'une bonne qualitétechnique obtenues pour cette région. Le moment dipolaire virtuel moyen leur étant associé(6.15+-2.1 10e22 Am2), est plus élevé que la valeur précédemment estimée pour l'oligocène.Néanmoins, il reste bien inférieur à la valeur moyenne obtenue pour les périodes récentes( 0.3 Ma), qui semblent anormalement élevées. D'autre part, le travail réalisé au cours de cette thèse contribue aux recherches géologiquesmenées à Kerguelen en apportant des contraintes propres au paléomagnétisme. A partird'études de la fabrique magnétique, une meilleure localisation des centres éruptifs des dif-férentes provinces volcaniques ainsi qu'une chronologie détaillée de la mise en place d'unesection de la partie Sud-Est de l'archipel, sont proposées. Le report sur une échelle magnéto-stratigraphique globale des successions de polarité observées pour cinq sections a permis parailleurs de préciser leurs intervalles de temps de mise en place et réviser ainsi l'estimation deleurs taux d'éruption respectifs

Fluctuations du champ paléomagnétique terrestre enregistrées par les laves de l'Archipel des Kerguelen (partie australe de l'Océan Indien) et détermination de leur direction de mise en place

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Post-emplacement tilting of lava flows inferred from magnetic fabric study: the example of Oligocene lavas in the Jeanne d’Arc Peninsula (Kerguelen Islands)

International audienc

Origin and age of the directions recorded during the Laschamp event in the Chaîne des Puys (France)

International audienceExcursions of the geomagnetic field are likely even more frequent than reversals and thus must be seen as a major characteristic of the geodynamo. The Laschamp event discovered in lava flows of Massif central (France) is the youngest and the most studied field excursion. Its geomagnetic origin has been controversial due to the existence of self-reversal processes. Taking advantage of new dated sites, the initial aim of this work was to find additional flows with intermediate or reverse directions but also to investigate the magnetic properties of normal polarity flows surrounding the event. This study combines thermal and alternating field demagnetization of 272 samples from 21 units including 12 new localities and 12 K–Ar datings of main units. No new site with intermediate or reverse polarity was found. Ten sites have a normal polarity and all sites studied at Olby, Louchadière and Royat display intermediate but scattered directions. Magnetic mineralogy is characterized by primary titanomagnetite, variable amounts of titanomaghemite and almost pure magnetite. We confirmed that reverse polarity flows are affected by self-reversals but we found that this is also the case for normal flows. A direct consequence is that self-reversals cannot be taken as responsible for the reverse directions but they likely contribute to generate dispersion. However the overall coherency of the directions indicates that this process was limited and despite complex magnetization, the geomagnetic origin of the Laschamp in the Chaîne des Puys is not questioned. All volcanic pole positions (VGPs) published so far show significant scatter but they remain consistent with each other. Interestingly, they do not coincide with the longitudinal loops seen in the sedimentary records and are compatible with a dominant non-dipolar field geometry. New K–Ar datings provide a coherent chronology of the successive polarities. The 37 ka old reverse directions of the Olby flow are chronologically consistent with the 41.9 kyr old normal polarity flows preceding the event. The Royat flow is only marginally intermediate and indicates that the end of the event was not younger than 33.3 ka in the Chaîne des Puys

Secular variation of the geomagnetic dipole during the past 2000 years

International audienceWe have constructed a very simple model of a time-varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe. They were also compared with the Hongre et al. (1998) time-varying spherical harmonic model and with the CALS7K-2 model by Korte et al. (2005). We find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. In a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to 1000 years ago, and the dipole was actually increasing from 0 until A.D. 500. The dipole moment of CALS7K is lower than the present estimate, probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data. The tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at Paris during the past 2 ka. The results show that the "archeomagnetic jerks" are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field